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  <title>Module: ActiveRecord::Locking::Pessimistic</title>
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          <td><strong>Module</strong></td>
          <td class="class-name-in-header">ActiveRecord::Locking::Pessimistic</td>
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            <td><strong>In:</strong></td>
            <td>
                <a href="../../../files/vendor/rails/activerecord/lib/active_record/locking/pessimistic_rb.html">
                vendor/rails/activerecord/lib/active_record/locking/pessimistic.rb
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      <p>
<a href="Pessimistic.html">Locking::Pessimistic</a> provides support for
row-level locking using SELECT &#8230; FOR UPDATE and other lock types.
</p>
<p>
Pass <tt>:lock =&gt; true</tt> to <a
href="../Base.html#M001674">ActiveRecord::Base.find</a> to obtain an
exclusive lock on the selected rows:
</p>
<pre>
  # select * from accounts where id=1 for update
  Account.find(1, :lock =&gt; true)
</pre>
<p>
Pass <tt>:lock =&gt; &#8216;some locking clause&#8216;</tt> to give a
database-specific locking clause of your own such as &#8216;LOCK IN SHARE
MODE&#8217; or &#8216;FOR UPDATE NOWAIT&#8217;.
</p>
<p>
Example:
</p>
<pre>
  Account.transaction do
    # select * from accounts where name = 'shugo' limit 1 for update
    shugo = Account.find(:first, :conditions =&gt; &quot;name = 'shugo'&quot;, :lock =&gt; true)
    yuko = Account.find(:first, :conditions =&gt; &quot;name = 'yuko'&quot;, :lock =&gt; true)
    shugo.balance -= 100
    shugo.save!
    yuko.balance += 100
    yuko.save!
  end
</pre>
<p>
You can also use ActiveRecord::Base#lock! method to lock one record by id.
This may be better if you don&#8216;t need to lock every row. Example:
</p>
<pre>
  Account.transaction do
    # select * from accounts where ...
    accounts = Account.find(:all, :conditions =&gt; ...)
    account1 = accounts.detect { |account| ... }
    account2 = accounts.detect { |account| ... }
    # select * from accounts where id=? for update
    account1.lock!
    account2.lock!
    account1.balance -= 100
    account1.save!
    account2.balance += 100
    account2.save!
  end
</pre>
<p>
Database-specific information on row locking:
</p>
<pre>
  MySQL: http://dev.mysql.com/doc/refman/5.1/en/innodb-locking-reads.html
  PostgreSQL: http://www.postgresql.org/docs/8.1/interactive/sql-select.html#SQL-FOR-UPDATE-SHARE
</pre>

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    <div id="method-list">
      <h3 class="section-bar">Methods</h3>

      <div class="name-list">
      <a href="#M001305">lock!</a>&nbsp;&nbsp;
      </div>
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      <h3 class="section-bar">Public Instance methods</h3>

      <div id="method-M001305" class="method-detail">
        <a name="M001305"></a>

        <div class="method-heading">
          <a href="#M001305" class="method-signature">
          <span class="method-name">lock!</span><span class="method-args">(lock = true)</span>
          </a>
        </div>
      
        <div class="method-description">
          <p>
Obtain a row lock on this record. Reloads the record to obtain the
requested lock. Pass an SQL locking clause to append the end of the SELECT
statement or pass true for &quot;FOR UPDATE&quot; (the default, an
exclusive row lock). Returns the locked record.
</p>
          <p><a class="source-toggle" href="#"
            onclick="toggleCode('M001305-source');return false;">[Source]</a></p>
          <div class="method-source-code" id="M001305-source">
<pre>
    <span class="ruby-comment cmt"># File vendor/rails/activerecord/lib/active_record/locking/pessimistic.rb, line 71</span>
71:       <span class="ruby-keyword kw">def</span> <span class="ruby-identifier">lock!</span>(<span class="ruby-identifier">lock</span> = <span class="ruby-keyword kw">true</span>)
72:         <span class="ruby-identifier">reload</span>(<span class="ruby-identifier">:lock</span> =<span class="ruby-operator">&gt;</span> <span class="ruby-identifier">lock</span>) <span class="ruby-keyword kw">unless</span> <span class="ruby-identifier">new_record?</span>
73:         <span class="ruby-keyword kw">self</span>
74:       <span class="ruby-keyword kw">end</span>
</pre>
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